Electric motor control circuit and switching means

ABSTRACT

An electric motor control circuit for selectively connecting and disconnecting a pair of input terminals of an armature of an electric motor to a source of electrical power to selectively brake and reverse the rotation of the armature. Switching means associated with the control circuit directs current to a first terminal of the armature to cause a selected direction of rotation and when braking is desired the first terminal is disconnected from the power source, while the second of the armature terminals is grounded directly or through a resistor to provide a dynamic braking without the application of an external voltage being applied across the pair of terminals of the armature. The switching means is so arranged that neither terminal of the armature can be prematurely grounded during the braking or reversing operations.

ited States Patent Inventor Thomas B. Dalton Muskegon, Mich. Appl. No.2,259 Filed Jan. 12, 1970 Patented July 13, I971 Assignee WestranCorporation,

Muskegon, Mich.

ELECTRIC MOTOR CONTROL CIRCUIT AND SWITCHING MEANS 2,285,666 6/1942 KingABSTRACT: An electric motor control circuit for selectively connectingand disconnecting a pair of input terminals of an armature of anelectric motor to a source of electrical power to selectively brake andreverse the rotation of the armature. Switching means associated withthe control circuit directs current to a first terminal of the armatureto cause a selected direction of rotation and when braking is desiredthe first terminal is disconnected from the power source, while thesecond of the armature terminals is grounded directly or through aresistor to provide a dynamic braking without the application of anexternal voltage being applied across the pair of terminals of thearmature. The switching means is so arranged that neither terminal ofthe armature can be prematurely grounded during the braking or reversingoperations.

PATENIEUJumm: 35930 4 SHEET 1 (1F 2 INVENTOR THOMAS B. DALTON BY ua,fiW; Aim,

ATTORNEYS PATENIEUJULI 3mm 3,593,084

SHEET 2 UF 2 FIG. 5

INVENTOR THOMAS B. DALTON ATTORNEYS ELECTRIC MOTOR CONTROL CIRCUIT ANDSWITCHING MEANS The present invention relates to electric motor controlcircuits, and particularly to motor reversing and dynamic brakingsystems. 2. Description of the Prior Art In present day motor reversingand dynamic braking systems it has been a common practice to dynamicallybrake vehicles and the like employing prime mover electric drives bydisconnecting the driving motor from the prime mover driven generatorand then connecting one of the driving motor terminals to ground wherebythe driving motor functions as a generator driven from an axle of thevehicle through the usual gearing, with suitable resistance dissipatingthe electrical energy thus developed. During the operation of thedriving motor as a generator, the field of the prime mover drivengenerator is reduced to a minimum commensurate with no load or idlingoperation and the braking effect is varied by adjusting the field of thedriving motor. This known procedure efi'ectively brakes the vehicle orany other prime mover electric driven motor so utilized.

Such reversing and dynamic braking systems usually employ, in typicalpractice, a double-pole, double-throw current reversing switch inconjunction with two or more diodes or resistors to provide the dynamicbraking. Although great skill and ingenuity have been exercised indesigning these systems to eliminate unsafe operation, they are notfoolproof, as some of the actuating mechanisms, such as solenoids whichare typically used, may fail. During the operation of an electric motorif one of the elements in the system fails, the terminals of thearmature may be accidentally grounded. In the event one of the terminalsof the armature of such an electric motor is grounded, the results canbe extremely hazardous to the operator or other individuals nearby asthe same may very well explode. Therefore, it would be desirable toprovide a motor reversing and dynamic braking system which has afail'safe switching mechanism to insure that neither of the terminals ofthe armature of the electric motor are prematurely grounded.

SUMMARY OF THE PRESENT INVENTION The present invention, which will besubsequently described in greater detail, comprises a control circuitfor the dynamic braking and reversing of an electric motor in whichpower of a fixed polarity is applied in determinable opposite directionsthrough first and second set of contactors or switches to apply areversible polarity voltage across the armature of a direct currentmotor which is desired to be controlled. The first and second sets ofcontactors or switches which control the current flow and hence thepolarity at the armature are in turn actuated by first and secondsolenoid coils, respectively, to cause the motor to rotate in aselective direction. In a neutral position, for example, both first andsecond solenoid coils are in a deenergized state, and the armature ofthe motor is grounded through the contactors or switches to providedynamic braking and no voltage is externally applied thereacross. Anovel switch actuated by the movements of the first and/or secondsolenoid coils insures that neither of the terminals of the armature ofthe motor is prematurely grounded.

It is therefore an object of the present invention to provide a simple,reliable, comparatively inexpensive electrical control circuit forcontrolling the direction of rotation of a direct current motor and thedynamic braking thereof.

Other objects, advantages and applications of the present invention willbecome apparent to those skilled in the art when the accompanyingdescription of one example of the best mode contemplated for practicingthe invention is read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The description herein makes referenceto the accompanying drawings wherein like reference numerals refer tolike parts, and in which:

FIGS. I, 2, 3 and 4 illustrate schematic circuit diagrams of a preferredembodiment of an electric motor control circuit;

FIG. 5 is a front elevational view of one embodiment of the presentinvention;

FIG. 6 is a top elevational view, partially in cross section, of theembodiment illustrated in FIG. 5;

FIG. 7 is a sectional view of the embodiment illustrated in FIG. 5 andtaken along the line 7-7 thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT outputs of the two fields aregrounded at 20 through the motor armature coils.

The relay It) includes a coil 22, normally closed contacts 23 and 2d, acontactor 25, and normally open contacts 26 and 28. Similarly, relay 12includes a coil 30, normally closed contacts 32 and 34 connected bymeans of contactor 36, and normally open contacts 38 and 40. When eitherof the coils 22 or 30 is energized, the contactor 25 or 36 is moved toopen the normally closed contacts 23 and 24 or 32 and 34 respectivelyand close the contacts 26 and 28 or 38 and 40. Coils 22 and 30 of therelays l0 and 12, respectively, are connected through a three-positionswitch generally indicated at 42 so that either one, but not both, maybe energized by means of the direct current power from source 14 toground through the switch. In particular, the terminals of the coils 22and 30 are respectively connected to the contacts 28 and 38, which, inturn, are connected to the power source 14 at a common juncture 44. Theother terminals of the coils 22 and 30 are respectively connected to thefixed contacts 46 and 48 of the three-position switch 42. The switch 42has a movable contact 50 which is grounded at 52 and is movable toengage either the contact 56 or 48, or be positioned at a neutral pointisolated from both contacts 46 and 48. By contacting the movable contact50 with either the contacts 46 or 48, one of the coils 22 or 30 will beenergized.

The input terminal 17 of the clockwise rotation field coil 15 v and theinput terminal 18 of the counterclockwise rotation field coil 16 arerespectively connected to the normally open co;.tacts 40 and 26 of therelays l2 and 10, respectively. Thus, when the contactors 2'5 and 36 areselectively actuated by the coils 22 and 30, current will flow from thecurrent power source 14 through the contacts 26 and 28 or 38 and 40,respectively, to their associated rotation field coils 16 or 15. Ashereinbefore indicated, the switch 42 can be actuated so as to energizeonly one coil 22 or 30 at one time, but not both at the same time, thuscurrent from the power source 14 can be directed to only one rotationfield coil 15 or 16 at one time.

The contact 23 is grounded at 54 through a variable resistor 56, whilethe contact 24 is connected to the contact 32 of the relay 12. Thecontact 23 may alternately be grounded directly or through a fixedresistor. Contact 34 of the relay 12 is, in turn, connected to atwo-position movable arm member 58 of a two-position switch 60. Themovable arm member 58 of the switch 60 is shiftable between a firstposition in which it engages a contact 62, which is connected to thecontact 26 of the relay I0, and a second position in which the armmember 58 engages a contact 64 which, in turn is connected to thecontact 40 of the relay 12. As will be explained in greater detailhereinafter in the description of the switch 60, the movable arm member58 is adapted to contact only one of the fixed contacts 62 or 64 at onetime and thus connect only one of the contacts 62 or 64 with the contact34.

The contactors 25 and 36 of the relays I and 12, respectively, areactuated to open and close their associated contacts by the relayarmatures 66 and 68. These same armatures 66 and 68 also actuate themovable arm member 58 of the switch 60. When the relay is actuated sothat the contactor 25 opens the contact 23 and 24 and closes the contact26 and 28, the armature 66 thereof engages the movable arm member 58 ofthe switch 60 and causes the same to engage the fixed contact 64 (FIG.4), and, when the coil 30 is energized so as to shift the contactor 36to open the contacts 32 and 34 and close the contacts 38 and 40, thearmature 68 thereof engages the movable arm member 58 of the switch 60and causes the same to disengage contact 64 and engage the contact 62(FIG. 2). It should be noted that when either of the relays 10 or 12 aredeenergized so that the same move to their normal position, that isengaging the contacts 23 and 24 or 32 and 34, the movable arm member 58is not moved, as the same is only moved upon the energizing of one ofthe relays and contact thereof by their respective armatures 66 or 68.

Referring now to the schematic diagram illustrated in FIG. 2 for adescription of the motor control circuit and how the same is activatedso as to provide current from the direct current power source 14 to thefield coil 15 to rotate the motor armature 19 in a clockwise direction,the movable contact 50 of the switch 42 is shifted to engage the fixedcontact 48, thus power is directed from the power source 14 through thejuncture 44, fixed contact 38, the coil 30 of the relay 12, and toground 52 by way of the switch 42. Thus, the coil 30 of the relay 12 isenergized to cause the armature 68 thereof to move downwardly, as viewedin the schematic diagrams, such that the contactor 36 disengages thenormally closed contacts 32 and 34 and engages the normally opencontacts 38 and 40 to close the same, while at the same time thearmature 68 of the relay 12 engages the movable contact 58 to shift thesame into engagement with the fixed contact 62. With the relay 12energized, current flows from the direct current power source 14 throughthe juncture 44, closed contacts 38 and 40, to the terminal l7, andthrough the field coil 15 to cause a clockwise rotation of the motorarmature 19. The solenoid coil 22 of the relay 10 cannot be actuatedsince the movable contact of the switch 42 can engage one contactthereof at a time, thus the power from the power source 14 is preventedfrom being transmitted to the coil 16 of the motor by the normally opencontacts 26 and 28 of the relay 10.

When it is desired to dynamically brake the motor armature 19, themovable contact 50 of the switch 42 is moved to its neutral position,thereby deenergizing the coil 30 of the relay 12, whereupon a spring(not shown) within the relay 12 moves the armature 68 and thus thecontactor 36 back into engagement with the contacts 32 and 34, therebybreaking the direct current flow between the source 14 and the fieldcoil 15 (see FIG. 3). When the armature 68 has been shifted to itsoriginal position by the deenergizing of the coil 30, the movable armmember 58, as viewed in FIG. 3, remains in its position, that is, inengagement with contact 62. With the motor armature rotating in aclockwise direction, an electromotive force is generated in the fieldcoil 16. The current generated thereby is dissipated through theresistor 56. It can be seen from FIG. 3 that a flow path for dissipationof the current from the field coil 16 is created between the terminal 18of the motor, fixed contact 26, the engagement of the contact 62 and themovable arm member 58 of the switch 60, which in turn is connected tothe resistor 56 through the normally closed contacts 23, 24, 32 and 34of the solenoid relays l0 and 12. The dynamic braking will slow down themotor 19 until it stops. The rate at which the motor 19 is slowed orstopped will depend on the amount of resistance in the resistor 56. Ifthe resistance is large, only a small amount of current will flowthrough the motor and only a small amount of electromagnetic reactionwill occur, and hence a relatively small braking force will be exertedon the motor armature 19. As the resistor 56 is decreased in itseffective value, there is a corresponding increase in magnitude of thecurrent flowing in the circuit for any given speed or rotation of thearmature of the motor and hence providing for an increased amount ofelectromagnetic reaction, and thus an increased braking force is exertedupon the motor armature 19. In selected applications, a fixed resistormay be used instead of the variable resistor 56.

In order to drive the electric motor armature 19 in a counterclockwisedirection, it is necessary to direct current from the power source 14 tothe field coil 16. This is accomplished by moving the movable contact 58of the switch 42 into engagement with the fixed contact 46 so as toenergize the coil 22 of the relay 10, whereupon, the armature 66 thereofmoves contactor 25 into engagement with the normally open contact 26 and28 to close the same, while opening the normally closed contacts 23 and24.

As can be seen in FIG. 4, the armature 66 of the relay 10 engages themovable arm member 58 of the switch 60 and moves the same intoengagement with the fixed contact 64. Thus, current from the powersource 14 is transmitted through the contacts 28, the contactor 25,contact 26, terminal 18 and the counterclockwise field coil 16 to rotatethe armature of the motor in a counterclockwise direction.

When braking or stopping of the motor is desired, the movable contact 50of the switch 42 is moved to its neutral position thereby deenergizingthe coil 22. The contactor 25 is moved to its normal position by springs(not shown) into engagement with the normally closed contacts 23 and 24,as illustrated in FIG. 1, while the movable arm member 58 of the switch60 remains in engagementwith the contact 64. Thus, the electromagneticforce generated by the current flowing through the field coil 15 isdissipated through the contacts 40 and 64, the switch 60, normallyclosed contacts 32 and 34 of the relay 12, the normally closed contacts23 and 24 of the relay l0, and to ground 54 through the resistor 56. Inthe same manner as hereinbefore described in the description of thebraking of the clockwise rotation of the motor armature 19, the rate ofbraking can be controlled by varying resistor 56.

Referring now to FIGS. 5, 6 and 7 for a detailed description of theswitch 60, it is to be understood that the switch 60 was only illustratgd schematically in FIGS. 1-4 and that the same consists of morecomponents than were shown therein.

Switch 60 is adapted to be mounted on a plate 69 made of electricalinsulating materia which, in turn, is bolted to the electrical relayassembly by screws 71., such that the output shaft '72 of a gear reducer73 on motor 19 extends through a half-round slot 74 on one edge of theplate 69. The switch 68 comprises a pair of flexible members 62 and 64(which correspond to the contacts 62 and 64 of FIGS. 1-4), one end ofeach being attached to the insulating plate 69 by rivets 75. Theflexible members 62 and 64, respectively, have movable contacts 76 and78 on their opposite free ends. The flexible members 62 and 64 are ofsufficient resiliency so that the contacts 76 and 68 are normally biaseddownwardly into engagement with fixed contacts 80 and 82 mounted on topof a transverse metal plate 84 which, in turn, is attached to theinsulating plate 69 by means of rivet 86. Movable arm member 58 isrotatably mounted to the insulating plate 69 by bushing 98 and nut andbolt assembly 92. Bushing insures free movement of the arm member 58 andis made of a tough insulating material which has self-lubricatingcharacteristics, such as nylon or the like. The nut and bolt assembly 92also fastens to the plate 69, a lead wire 94 and a connecting plate 96;the connecting plate 96 connecting the lead wire 94 to transverse plate84 and thus to the fixed contacts 80 and 82, while lead wire 94 isconnected to the normally closed contact 34 illustrated. in theschematic diagrams in FIGS. l-4. The opposite ends of flexible members62 and 64 are respectively connected directly to the normally opencontacts 26 and 40, also illustrated in FIGS. 1--4.

The arm member 58, which is also made of a nonelectrical conductingmaterial, is adapted to rotate in both a clockwise and counterclockwisedirection (as viewed in FIG. 5) in response to the relay armatures 66and 68, respectively. Thus, when the coil 22 of the relay is energizedso as to drive the armature 66 into engagement with the arm member 58,the armature 66 causes the arm member 58 to rotate in a counterclockwisedirection whereupon an end 102 of the arm member 58 abuts a recessedportion 104 formed in the flexible member 62 to raise the same upwardlysuch that the contact 76 separates from the contact 80 thus breaking theconnection between the contact 26 and the normally open contact 40.Since the other flexible member 64 is normally biased downwardly, themovable contacts 78 carried on its free end thereof remain engaged withthe contact 82, thus the flexible member 64 is connected to the normallyopen contact 40 in the same manner as hereinbefore described in thedescription of the schematic drawings in FIGS. 1-4. When relay 10 isdeenergized, the armature 66 will move away from the arm member 58.However, the arm. .nember end 102 remains in contact with the recess 104to prevent engagement between the contacts 76 and 80. When the coil 30of the relay 12 is energized, the armature 68 is shifted into contactwith the movable arm member 58 whereupon its end 105 thereof engages arecess 106 formed in the flexible member 64 to break the electricalconnection between the contacts 78 and 82, while at the same time theend 102 of the arm member 58 is rotated clockwise away from the recess104 of the flexible member 102 to permit the contact 76 carried on thefree end thereof to engage the fixed contact 80. The circuit will thenfunction in the same manner as herein described in the schematicdrawings of FIGS. 1-4.

It can thus be seen that the present invention has provided an electricmotor control system for the dynamic braking and reversing of anelectric motor in a simple and reliable manner, in which the means forgrounding the rotation field coils of the electric motor is such as toprovide a fail-safe means for preventing a premature grounding andpotential hazard to the electric motor and users thereof.

Having thus described the invention, what i claim is as follows:

1. An electrical motor control circuit comprising:

a source of power of a fixed polarity;

first and second sets of contactors;

a motor having an armature and field coils, said first and second setsof contactors applying a reversible polarity voltage across said fieldcoils to control the direction for current flow and thus the polarity atsaid armature;

first and second actuating means selectively movable between first andsecond positions to respectively actuate said first and second sets ofcontactors to cause said armature to rotate in a selected direction whenin said first position, and brake rotation of said armature when in saidsecond position;

a switching means operable in response to the position of said actuatingmeans to move between first and second positions, one of said actuatingmeans positioning said switching means to said first switching meansposition when said one actuating means is moved to its first position,said switching means remaining in its first position to electricallyground said armature and field coils when said one actuating means ismoved to its second position;

the other of said actuating means positioning said switching means tosaid second switching means position when said other actuating means ismoved to its first position, said switching means remaining in itssecond position to electrically ground said armature and field coilswhen said other actuating means is moved to its second position.

2. The electrical motor control circuit as defined in claim 1 whereinsaid armature and field coils are grounded through a resistor.

3v The electrical motor control circuit as defined in claim 2 whereinsaid resistor is a variable resistor.

4. An electrical motor control circuit comprising:

a source of electrical power;

an electrical motor including an armature and field coils having a pairof input terminals; means for selectively connecting said terminals tosaid source of electrical power for selectively rotating said armaturein opposite directions and for disconnecting said source from saidterminals to stop said rotation; said last mentioned means comprising:

first and second switch means each including first and second normallyclosed contacts, first and second normally opened contacts, and a thirdcontact electrically connected to said first normally opened contact;said second normally opened contact of said first switch means and saidfirst normally opened contact of said second switch means beingrespectively connected to said source of electrical power; said firstnormally opened contact of said first switch means and said secondnormally opened contact of said second switch means being respectivelyconnected to said pair of input terminals;

said first and second normally closed contacts of said first switchmeans being respectively grounded and connected to said first normallyclosed contact of said second switch means;

a third switch means having a contact member movable between first andsecond positions respectively engaging said third contacts of said firstand second switch means;

first actuating means being operable between first and second positionsto respectively open and close said first and second normally closedcontacts of said first switch means and close and open said first andsecond normally opened contacts of said first switch means while movingsaid third switch means into its first position when said firstactuating means is in its first position, said third switch meansremaining in its first position when said first actuating means isreturned to its second position; and

second actuating means being operable between first and second positionsto respectively open and close said first and second normally closedcontacts of said second switch means and close and open said first andsecond normally closed contacts of said second switch means, whilemoving said third switch contact to its second position when said secondactuating means is in its first position, said movable contact memberremaining in its second position when said second actuating means isreturned to its second position.

5. The electrical motor control circuit as defined in claim 4 whereinsaid third switch means comprises:

an electrically nonconducting mounting plate;

first and second flexible members, the opposite ends of each of saidflexible members being carried by said plate and having first and secondmovable contacts respectively carried on their free end;

rirst and second fixed contacts carried on said plate, said flexiblemembers being of sufficient resiliency that said first and secondmovable contacts are respectively normally biased into engagement withsaid first and second contacts;

an arm member mounted at its midsection on said plate at a point spacedfrom said first and second flexible members, said arm member beingrotatable between first and second positions respectively in response tosaid first and second actuating means and so arranged that when said armmember is rotated in one direction in response to one of said actuatingmeans said arm member engages the underside of one of said flexiblemembers to raise the movable contact carried thereby away from itsassociated fixed contact;

said arm member engaging the underside of the other of meanselectrically connecting one of said movable contacts to one of saidinput terminals of said armature and for 7 8 electrically connecting theother of said movable contacts 6. The electrical motor control circuitas defined in claim to the other of said input terminals of saidarmature; and wherein said armature is grounded through a resistor.means for electrically connecting said fixed contacts to said Anelectrical control circuit as defined in claim 6 second normally closedcontact of said second switch wherein Said resistor is a variableresistormeans.

1. An electrical motor control circuit comprising: a source of power ofa fixed polarity; first and second sets of contactors; a motor having anarmature and field coils, said first and second sets of contactorsapplying a reversible polarity voltage across said field coils tocontrol the direction for current flow and thus the polarity at saidarmature; first and second actuating means selectively movable betweenfirst and second positions to respectively actuate said first and secondsets of contactors to cause said armature to rotate in a selecteddirection when in said first position, and brake rotation of saidarmature when in said second position; a switching means operable inresponse to the position of said actuating means to move between firstand second positions, one of said actuating means positioning saidswitching means to said first switching means position when said oneactuating means is moved to its first position, said switching meansremaining in its first position to electrically ground said armature andfield coils when said one actuating means is moved to its secondposition; the other of said actuating means positioning said switchingmeans to said second switching means position when said other actuatingmeans is moved to its first position, said switching means remaining inits second position to electrically ground said armature and field coilswhen said other actuating means is moved to its second position.
 2. Theelectrical motor control circuit as defined in claim 1 wherein saidarmature and field coils are grounded through a resistor.
 3. Theelectrical motor control circuit as defined in claim 2 wherein saidresistor is a variable resistor.
 4. An electrical motor control circuitcomprising: a source of electrical power; an electrical motor includingan armature and field coils having a pair of input terminals; means forselectively connecting said terminals to said source of electrical powerfor selectively rotating said armature in opposite directions and fordisconnecting said source from said terminals to stop said rotation;said last mentioned means comprising: first and second switch means eachincluding first and second normally closed contacts, first and secondnormally opened contacts, and a third contact electrically connected tosaid first normally opened contact; said second normally opened contactof said first switch means and said first normally opened contact ofsaid second switch means being respectively connected to said source ofelectrical power; said first normally opened contact of said firstswitch means and said second normally opened contact of said secondswitch means being respectively connected to said pair of inputterminals; said first and second normally closed contacts of said firstswitch means being respectively grounded and connected to said firstnormally closed contact of said second switch means; a third switchmeans having a contact member movable between first and second positionsrespectively engaging said third contacts of said first and secondswitch means; first actuating means being operable between first andsecond positions to respectively open and close said first and secondnormally closed contacts of said first switch means and close and opensaid first and second normally opened contacts of said first switchmeans while moving said third switch means into its first position whensaid first actuating means is in its first position, said third switchmeans remaining in its first position when said first actuating means isreturned to its second position; and second actuating means beingoperable between first and second positions to respectively open andclose said first and second normally closed contacts of said secondswitch means and close and open said first and second normally cloSedcontacts of said second switch means, while moving said third switchcontact to its second position when said second actuating means is inits first position, said movable contact member remaining in its secondposition when said second actuating means is returned to its secondposition.
 5. The electrical motor control circuit as defined in claim 4wherein said third switch means comprises: an electrically nonconductingmounting plate; first and second flexible members, the opposite ends ofeach of said flexible members being carried by said plate and havingfirst and second movable contacts respectively carried on their freeend; first and second fixed contacts carried on said plate, saidflexible members being of sufficient resiliency that said first andsecond movable contacts are respectively normally biased into engagementwith said first and second contacts; an arm member mounted at itsmidsection on said plate at a point spaced from said first and secondflexible members, said arm member being rotatable between first andsecond positions respectively in response to said first and secondactuating means and so arranged that when said arm member is rotated inone direction in response to one of said actuating means said arm memberengages the underside of one of said flexible members to raise themovable contact carried thereby away from its associated fixed contact;said arm member engaging the underside of the other of said flexiblemembers when rotated in an opposite direction to raise the movablecontact carried thereby away from its associated fixed contact; meanselectrically connecting one of said movable contacts to one of saidinput terminals of said armature and for electrically connecting theother of said movable contacts to the other of said input terminals ofsaid armature; and means for electrically connecting said fixed contactsto said second normally closed contact of said second switch means. 6.The electrical motor control circuit as defined in claim 5 wherein saidarmature is grounded through a resistor.
 7. An electrical motor controlcircuit as defined in claim 6 wherein said resistor is a variableresistor.